1. Field of the Invention
The present invention relates to improvements in a lithium secondary cell and a positive electrode material for a lithium secondary cell employing a spinel type lithium manganese oxide, particularly to improvements of the charge-discharge capacity, the cycle characteristics and the thermal stability of the cell.
2. Discussion of Background
As a positive electrode active material for a lithium secondary cell, LiMn.sub.2 O.sub.4 has been proposed which is a compound oxide of manganese and lithium and which has a spinel structure, and various studies are being made thereon. This compound has a high voltage-high energy density and, in addition, has a merit that it is inexpensive, as the reserve of manganese as the starting material is larger than the reserve of cobalt or nickel when compared with a double oxide of cobalt and lithium or a double oxide of nickel and lithium. On the other hand, it has problems that the charge-discharge cycle life is short, and it is inferior in high temperature stability.
With respect to an improvement of the cycle characteristics of such a lithium manganese oxide at room temperature, it has been proposed to modify the lithium manganese oxide, for example, by increasing lithium excessively to obtain Li.sub.1+x Mn.sub.2-x O.sub.4 as disclosed in JP-A-7-282798, or by substituting a part of manganese with other metals such as Co, Cr, etc., to obtain LiMn.sub.2-x Co.sub.x O.sub.4 or LiMn.sub.2-x Cr.sub.x O.sub.4, as disclosed in JP-A-3-108261 or JP-A-3-219571. However, such modification methods end up with a trade-off such that the initial charge-discharge capacity decreases in proportion to the substitution ratio. The reason is considered to be such that as a result of the substitution by a metal having a valency of at most 3, such as Li, Co or Cr, the average valency of Mn has increased on the principle of electrical neutrality, whereby the proportion of Mn.sup.3+ ions contributing to the charge-discharge reaction has decreased.
Under these circumstances, JP-A-9-82362 proposes to substitute a part of Mn by a cation having an oxidation number lower than Mn thereby to limit the substitution ratio so as to limit the oxidation number in the initial state of the transition metal and to realize a high charge-discharge capacity and excellent cycle characteristics simultaneously.
On the other hand, with respect to improvement of the thermal stability, various studies have been made and reported for the purpose of improving cycle characteristics at a high temperature or improving the storage characteristics. For example, Journal of Power Sources, 74(1998) 228-233, discloses one having a part of Mn substituted by Co, and Electrochemical Society Proceedings, volume 97-18.494, discloses one having a part of Mn substituted by Co or one having a part of oxygen substituted by F, and results are shown which indicate that they have effects for improving high temperature cycle characteristics. However, such results are nothing more than the interrelation for the trade-off from the viewpoint of the balance with the initial charge-discharge capacity. Further, JP-A-8-264183 proposes to form a coating film of a metal fluoride on the surface of the active material, and JP-A-8-213014 proposes to improve the storage characteristics at a high temperature by carrying out surface modification by e.g. fluorination treatment of an active material. However, such attempts are likely to bring about a trouble in introduction or discharge of Li ions which takes place at the time of charging and discharging.
As described above, the methods for improvement so far proposed have not been able to get out of the trade-off relation such that if it is attempted to improve one of the initial charge-discharge capacity, the cycle characteristics and the thermal stability, the rest may deteriorate. In order to break through such a situation, it has been desired to develop a novel concept and a lithium manganese oxide improved by such a concept.